Adrenergic beta-agonistic drugs characteristically contain as part of their structure an ethanolamine or 2-amino-ethanol moiety. Since the chemical structures of these drugs usually comprise at least one asymmetric carbon atom, these drugs commonly exist in optically active isomeric form, with the chiral carbon atom having (R) or (S) configuration. When there is a single asymmetric carbon atom present, the beta-receptor agonists exist as individual (R) or (S) enantiomers or in racemic (RS) form, i.e. as a 50:50 mixture of (R) and (S) enantiomers.
Compounds with two chiral centers have four isomers: the RR—, SS—, RS—, and SR-isomers. Such compounds (e.g. formoterol, ractopamine) may exist in a number of forms i.e. in the pure RR or SS or RS or SR isomeric forms, or as mixtures, hereinafter called “enantiomeric pairs” of either RR/SS or RS/SR. The compound ractopamine can also exist as racemic mixtures of all four isomers (RR+SS+RS+SR) or in the form of racemic mixtures of the enantiomeric pairs (RR/SS) or (RS/SR). The isomers (RR) and (SS) are mirror images of each other and are therefore enantiomers, which have the same chemical properties and melting points. (RS) and (SR) is similarly an enantiomeric pair. The mirror images of (RR) and (SS) are not, however, super imposable on (RS) and (SR). This relationship is called diastereomerism, and (RR) is a diastereomer to (RS).
Ractopamine has the molecular formula C18H23NO3 and racemic ractopamine is typically prepared as a hydrochloride salt. Chemically, ractopamine is differs from dobutamine in the location of only one hydroxyl group, but ractopamine is not a catecholamine and is therefore not instantaneously metabolised by catechol-O-methyl transferase. Ractopamine HCl (4-hydroxy-a-[[[3-(4-hydroxyphenyl)-1-methylpropyl]amino]methyl]benzenemethanol hydrochloride) has a molecular weight of 337.85 and a molecular formula of C18H23NO3.HCl (CAS number: 90274-24-1). The racemate of ractopamine is a mixture of all four isomers in approximately equal proportions.
